Isocyanate terminated prepolymers are commonly used to produce polyurethane and/or polyurea products like elastomers, foams, coatings, adhesives, sealants, caulks, and binders. However, the prepolymer manufacturing process can often result in high residual concentrations of the polyisocyanates used in the prepolymer synthesis. The residual polyisocyanate can lead to potential health and safety issues, and may also be detrimental to the performance and attributes of the end use product. For example, residual polyisocyanate can lead to undesired losses in open time, product instability, increased moisture sensitivity and decreased adhesion due to migration of these molecules to the interface. Polyurethane prepolymers that contain controlled levels of residual polyisocyanate, preferably below 1 wt %, can reduce health and safety risks and improve end product performance.
Since residual polyisocyanate can pose significant health and safety risks as well as reductions in product performance, a number of products and processes have been introduced that offer reduced residual polyisocyanate levels. A variety of methods are taught in the patented art for reducing the residual isocyanate content of polyisocyanate adducts: wiped film evaporation, solvent aided distillation/co-distillation, molecular sieves, and solvent extraction.
JP 08176252 discloses reacting MDI with straight chain molecule with MW 250-4000 and two active hydrogens at an equivalent ratio (NCO:OH) of 2.5-10:1. Free MDI is vacuum distilled to 1 wt % or less. The examples show polytetramethylene glycol (PTMEG) and ethylene glycol adipate.
U.S. Pat. No. 5,202,001 discloses preparing urethane prepolymers having low levels of residual organic diisocyanate. The examples show only prepolymers made from toluene diisocyanate TDI, isophorone diisocyanate IPDI and methylene-bis[(4-cyclohexyl)-diisocyanate] CHDI.
U.S. Pat. No. 5,703,193 discloses a process for reducing the amount of residual organic diisocyanate monomer in a polyurethane prepolymer reaction product by distilling in the presence of an inert solvent blend, one with boiling point above the monomer and one with boiling point below. Comparative examples show the removal of MDI monomer from a PTMEG 1000/MDI prepolymer reaction product.
U.S. Pat. No. 5,441,080 discloses a reactive hot melt composition comprising a polyester polyether thermoplastic elastomer and a polyisocyanate prepolymer. A MDI/PTMEG prepolymer is shown blended with a thermoplastic copolymer.
U.S. Pat. No. 6,133,415 discloses countercurrent extraction method for making polyurethane prepolymers. The examples show MDI/PTMEG prepolymers processed to give low free MDI.
U.S. Pat. No. 4,888,442 discloses a process for reducing the free monomer content of a polyisocyanate adduct by treating the adduct with 2-30 wt % inert solvent in an agitated thin-layer evaporator under conditions sufficient to reduce the free monomer content of the polyisocyanate adduct mixture below that level which is obtainable in the absence of a solvent. There are no examples showing the use of MDI as a suitable diisocyanate to prepare the polyisocyanate adduct.
U.S. Pat. No. 5,821,316 discloses an isocyanate-terminated polyurethane prepolymer comprising the reaction product of a polyisocyanate and a particular polyol composition. While the examples show the use of polypropylene oxide glycols, only TDI was used as the polyisocyanate.
EP 0 827 995 discloses hot melt adhesives comprising a polyisocyanate prepolymer prepared by reacting a polyisocyanate with a functionality of at least 2 with a polyol with a functionality of at least 2, the reaction product comprising at least 90 wt % “perfect” prepolymer and less than 2 wt % unreacted isocyanate monomer and the prepolymer having a free NCO functionality ranging from 0.2 to 8 wt %.
Airthane® prepolymers marketed by Air Products and Chemicals, Inc. are controlled structure prepolymer compositions. These prepolymer compositions which are based on TDI and IPDI contain >90 wt % perfect 2:1 prepolymer adducts, i.e., <10 wt % oligomer, and <0.1 wt % residual monomeric isocyanate.
We have found that 4,4′-diphenylmethane diisocyanate/polypropylene glycol (MDI/PPG) prepolymers low in free MDI monomer content can be synthesized which show performance advantages over conventional MDI/PPG prepolymers.